A device is known, such as that described in international patent application no. WO2005/094127, which makes it possible to localize the induction heating, so as to delimit the heating at the mold/material interface.
Such a device comprises inductors surrounding two electrically conductive mold bodies and comprising a heating zone intended to be in contact with the material to be shaped, the mold bodies being electrically insulated from each other. Thus, thanks to this electrical disconnection between the two mold bodies, the opposite faces of these latter delimit an air gap through which the magnetic field created by the inductors flows. The magnetic field thus induces electrical currents on the surface of the mold bodies, and especially on the surface of the heating zone of each mold body, thereby allowing the heating to be localized at the surface, close to the material to be heated.
Such a device allows a very quick and very significant rise in temperature of the heating zones, given the fact that the energy generated by the inductors is “injected” directly at the surface of the heating zones, in a very thin layer (typically a few tenths of a millimeter). To benefit as much as possible from the effect of the air gap, its width, i.e. the distance between the opposite faces of the device when it is operating, must be as small as possible, of the order of a few millimeters. In practice, this width is determined by the thickness of the part to be heated, which acts as an insulator between the two portions of the device. When this part is electrically conductive, insulating shims of a suitable thickness to insulate the two portions of the device, or an insulating coating on the surfaces in contact with the part, are provided.
Some materials require special molding techniques. This is the case, for example, for thermoplastic materials with long fibers, called L.F.T (“Long Fiber Thermoplastics”). To be properly molded, such material must be deposited hot onto a mold that is itself already at temperature. However, the known molds, because of their thermal inertia, do not permit heating/cooling cycles that are fast enough to be able to deposit material onto a mold at the ideal temperature and then cool this mold to obtain a solidified part, all in a commercially interesting time. To overcome this problem, the current techniques utilize molds maintained at a constant “intermediate” temperature, which is a compromise between the satisfactory flowing of the material and its correct solidification in the mold. At the same time, the material is deposited at a very high temperature, close to its degradation limit. For instance, for an L.F.T. material deposited at 250° C., the mold used will be at an intermediate temperature, between 80° C. and 100° C., which allows an acceptable flowing of the material, and at the same time its cooling below its solidification point.
To perform such an operation, it is known to carry out the preheating of the material outside of the mold, for example in an infrared oven or on a hot plate, then move the material onto a two-piece mold, the latter being kept at the required temperature while the material is preheated. The material is deposited in the mold as a soft and malleable paste that, under the pressure exerted by the two portions of the mold, begins to flow to fill the entire molding space, thus taking the shape of the finished part. To carry out this operation it is necessary that both portions of the mold, when in contact, define a compression chamber, i.e. that sealing is provided in order to exert the pressure necessary for the material to flow without it escaping. The temperature of the mold allows the gradual cooling of the material below its solidification point, so the part can be ejected. However, the mold temperature is often too high for optimum cooling, and the part is often still soft when removed, which poses problems with the final quality (distortion, residual stresses, etc.).
In summary, the methods currently implemented represent a compromise that does not allow either the satisfactory flowing of the material or sufficient cooling of the finished part to be achieved.